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Star: the story on HearLore | HearLore
Star
The nearest star to Earth is the Sun, yet it is merely one of an estimated two trillion stars scattered across the observable universe. A star is a luminous spheroid of plasma held together by its own gravity, shining because of the thermonuclear fusion of hydrogen into helium within its core. This process releases energy that traverses the star's interior and radiates into outer space, creating the light that has guided civilizations for millennia. The total mass of a star is the primary determinant of its evolution and eventual fate, dictating whether it will shine for billions of years or explode in a cataclysmic supernova. While the Sun appears as a fixed point of light to the naked eye, it is actually moving through space, and its immense distance from Earth makes it appear as a disk rather than a point, unlike the other stars which twinkle due to the Earth's atmosphere.
Ancient Sky Watchers
The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC, yet the history of stellar observation stretches back even further into the collective consciousness of humanity. Early astronomers recognized a difference between fixed stars, whose position on the celestial sphere does not change, and wandering stars, which are now known as planets. The Babylonian astronomers of Mesopotamia compiled the earliest known star catalogues in the late 2nd millennium BC during the Kassite Period, while the Greek astronomer Hipparchus created a catalog of 1,020 stars in the 2nd century BC. In 185 AD, Chinese astronomers were the first to observe and write about a supernova, now known as SN 185, challenging the ancient belief that the heavens were immutable. The brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and built the first large observatory research institutes to produce star catalogues, including the Book of Fixed Stars written by Abd al-Rahman al-Sufi in 964.
The Science of Light
In 1838, Friedrich Bessel made the first direct measurement of the distance to a star, 61 Cygni, using the parallax technique and proving that the stars were separated by vast distances. The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi, who found differences in the strength and number of absorption lines in the spectra of stars like Sirius compared to the Sun. Annie J. Cannon developed the modern version of the stellar classification scheme in the early 1900s, categorizing stars based on their spectra. In 1921, Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis, a revolutionary idea that was initially rejected by the scientific community. The development of the Hertzsprung-Russell diagram in 1913 propelled the astrophysical study of stars, allowing astronomers to model the interiors of stars and understand stellar evolution.
Common questions
What is the nearest star to Earth?
The nearest star to Earth is the Sun. It is one of an estimated two trillion stars scattered across the observable universe.
When was the oldest accurately dated star chart created?
The oldest accurately dated star chart was created in 1534 BC by ancient Egyptian astronomers. This record marks the beginning of documented stellar observation history.
Who made the first direct measurement of the distance to a star?
Friedrich Bessel made the first direct measurement of the distance to a star in 1838. He used the parallax technique to measure the distance to 61 Cygni.
How long will the Sun live before it becomes a red giant?
The Sun is expected to live 10 billion years. In about 5 billion years, it will enter the helium burning phase and expand to a maximum radius of roughly 1 AU.
What happens when a massive star exceeds 9 solar masses?
Massive stars exceeding 9 solar masses expand to form blue supergiants and then red supergiants. They eventually end their lives when their cores collapse and they explode as supernovae.
When did Chinese astronomers first observe a supernova?
Chinese astronomers were the first to observe and write about a supernova in 185 AD. This event is now known as SN 185 and challenged the ancient belief that the heavens were immutable.
Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores, a phase known as the main sequence. The Sun is expected to live 10 billion years, while massive stars consume their fuel very rapidly and are short-lived, lasting only a few million years. Low mass stars, called red dwarfs, consume their fuel very slowly and can last tens to hundreds of billions of years, with the most extreme of them lasting about 12 trillion years. When stars of at least 0.5 solar masses exhaust the supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core, causing the outer layers to expand and cool as they transition into a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 AU, 250 times its present size, and lose 30% of its current mass. Massive stars, those exceeding 9 solar masses, expand to form first a blue supergiant and then a red supergiant, eventually ending their lives when their cores collapse and they explode as supernovae.
The Final Remnants
If what remains after the outer atmosphere has been shed is less than roughly 1.4 solar masses, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. In massive stars, fusion continues until the iron core has grown so large that it can no longer support its own mass, causing the core to suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova, which may briefly outshine the star's entire home galaxy. The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster, or in the case of the largest stars, a black hole greater than 3 solar masses. The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars, allowing the formation of rocky planets and life itself.
Dancing in Gravity
Most stars are observed, and most or all may have originally formed in gravitationally bound, multiple-star systems, with 80% of very massive O and B class stars believed to be part of multiple-star systems. A multi-star system consists of two or more gravitationally bound stars that orbit each other, and the simplest and most common multi-star system is a binary star. When any star expands to become a red giant, it may overflow its Roche lobe, the surrounding region where material is gravitationally bound to it, and if stars in a binary system are close enough, some of that material may overflow to the other star. This mass transfer leads to cases such as the Algol paradox, where the most-evolved star in a system is the least massive, and can yield phenomena including contact binaries, common-envelope binaries, cataclysmic variables, blue stragglers, and type Ia supernovae. In a 2017 study of the Perseus molecular cloud, astronomers found that most of the newly formed stars are in binary systems, suggesting that all stars initially formed as binaries, though some binaries later split up and leave single stars behind.
Measuring the Cosmos
The surface temperature of a main-sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index. Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line, and was later reordered by temperature. The Sun is a main-sequence G2V yellow dwarf of intermediate temperature and ordinary size, while the most luminous known stars have absolute magnitudes of roughly -12, corresponding to 6 million times the luminosity of the Sun. The faintest red dwarfs in the NGC 6397 cluster are absolute magnitude 15, while a 17th absolute magnitude white dwarf has been discovered, representing the theoretical lower limit of mass at which stars are capable of supporting nuclear fusion of hydrogen in the core.
The Magnetic Dynamo
The magnetic field of a star is generated within regions of the interior where convective circulation occurs, functioning like a dynamo wherein the movement of electrical charges induce magnetic fields. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field, producing starspots which are regions of strong magnetic fields and lower than normal surface temperatures. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time, so that older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. During the Maunder Minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity, demonstrating that the activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.